In the field of physical distribution, an RFID system of attaching an RFID tag (hereinafter simply referred to as “tag”) to a mobile object such as a cargo and performing wireless communication between the tag and a tag communication device such as a reader/writer is recently adopted, so that cargo management is carried out by such an RFID system. According to the RFID system, the reader/writer is installed along conveying means such as a belt conveyor, and an interrogating wave is transmitted from the reader/writer. A response wave is then transmitted from the tag attached to the cargo being conveyed by the belt conveyor and the like and data such as ID (Identification) is automatically read, and hence physical distribution work becomes more efficient than the related art.
However, in such an RFID system, the reader/writer detects presence of the tag with only the transmission of the interrogating wave. Thus, in order for the tag and the reader/writer to communicate, the interrogating wave needs to be transmitted on a steady basis regardless of whether or not the tag is in a communication range of the reader/writer. It is a waste of power to have the reader/writer transmit the interrogating wave on a steady basis. If a plurality of reader/writers is installed and the number of channels assigned to each reader/writer is few, radio interference may occur between the reader/writers or tag confusion may occur in the communication with the tag when each reader/writer transmits the interrogating wave, whereby a possibility of non-communication becomes high.
To solve such a problem, a system in which the reader/writer transmits the interrogating wave only when the tag is in the communication range instead of transmitting the interrogating wave on a steady basis is proposed, where a mobile object detection method and a device described in Patent Document 1 are used for such a system.
The mobile object detection device described in Patent Document 1 temporarily interrupts the communication and does not perform read or write of data in a non-detecting area if the antenna includes one or more detecting areas and non-detecting areas. The disconnection control of the communication is carried out so that read and write of data are performed only in the detecting area. Furthermore, when carrying out the disconnection control of the communication, whether or not the tag is detected is verified, and whether or not to perform the read or write of data is determined. In such a mobile object detection device, whether the tag is detected is determined by tag detection means and the read and write of data are performed only in the detecting area instead of the interrogating wave being transmitted on a steady basis. As a result, a read and write error of the data originating from the non-detecting area can be effectively avoided, and unnecessary radio transmission can be reduced.
Although a specific configuration of the tag detection means is not described in Patent Document 1, the tag detection means is assumed to have a configuration equivalent to a sensor, with reference to the description of the specification and the drawings thereof. However, the system that detects the mobile object using the sensor and communicates with the tag leads to enlargement of the system and also results in high cost. In addition, the occupying band of the frequency becomes wide when the transmission speed becomes high, which may become the cause of interference between channels. To prevent interference to an adjacent channel, the transmission speed is desirably maintained at low speed as much as possible under a situation where high-speed communication is not required, but a criterion for maintaining a low speed is lacking.    Patent Document 1: Japanese Unexamined Patent Publication No. 2006-99189